Wireless cellular communications systems, e.g., IS-95 in the U.S., use code division multiple access (CDMA) to allow multiple users to share the same bandwidth. CDMA techniques offer many advantages, making them the preferred multiple access techniques for future cellular systems. Nevertheless, designers of these systems must contend with a significant amount of interference due to the non-orthogonal multiplexing of signals that results from the multipath induced by the channel, producing intersymbol interference (ISI).
The CDMA system forward link signal, that is, from base station to subscriber devices, consists of multiple users' signals transmitted synchronously by the base station. In current and near-term future CDMA systems, the subscriber device has knowledge only of its own spreading code. Therefore, blind interference suppression (IS) algorithms are required to cancel the multiple access interference (MAI).
Subscriber devices, such as those compliant with the IS-95 standard, employ a rake receiver for combining multipath components, but this technique fails in the context of a highly loaded CDMA system because the rake receiver treats the interfering users as additive white Gaussian noise (AWGN) and does not attempt to suppress them. For third generation wireless systems, i.e., CDMA2000 (U.S.) and WCDMA (Europe and Asia), more robust interference mitigation techniques are required to support the higher data rates, increased capacity requirements, and service requirements.
The study of interference suppression techniques has been an active area of research for the past few years and will continue to grow for third generation wireless communications systems and beyond. Several such techniques exist, some requiring training data to adapt, others focusing on data-aided or blind minimum mean square error (MMSE) receivers, since the interfering users' codes are unknown. However, no technique exists which can estimate the interfering users' spreading codes at the subscriber device, thereby enabling powerful interference cancellation or suppression algorithms.
What is needed is a code detection technique for CDMA systems that can quickly and accurately estimate the spreading codes of other user devices in a forward link signal in order to suppress multiple access interference.